Experimental Control of the Structure in SrCuO₂ Ultrathin Films
Ping-Chih Chiang1*, Ching-Shun Ku2, Jan-Chi Yang3, Ying-Hao Chu4
1Institute of Physics, National Chiao Tung University, Hsinchu, Taiwan
2National Synchrotron Radiation Research Center, Hsinchu, Taiwan
3Institute of Physics, National Cheng Kung University, Tainan, Taiwan
4Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan
* Presenter:Ping-Chih Chiang, email:pizejohn@yahoo.com.tw
A two-dimensional (2D) CuO₂plane plays a key role in high-Tc superconductivity. A recent theoretical study predicted a transition from the chain-type structure to the bulk plane-type structure in SrCuO₂films with thickness larger than five unit cells (uc), and the transition is due to the electrostatic instability [1]. In present study, ultrathin (1-20 uc) SrCuO₂ films were successfully grown on TiO₂-terminated SrTiO₃substrates using pulsed-laser-deposition technique. The thickness-dependent structural transformation of the SrCuO₂ films was studied by polarized X-ray absorption spectroscopy (XAS) at the Cu L-edge and Laue diffraction. The XAS results clearly distinguish the plane-type structure from the chain-type one. In the out-of plane absorption spectra, the spectral weight decreases with the increase of samples’ thickness. In the 7-uc films, the out-of-plane absorption spectra show the same behavior in the samples with the infinite-layer one. In 10-uc and 15-uc ultrathin films, the tetragonal-structure identification were characterized by synchrotron-based X-ray diffraction techniques. In Laue diffraction measurements, an extra diffraction peaks was observed in the no-STO-capping samples with thickness larger than 10 uc. By comparing different crystal structure models, those extra peaks indicate the plane-type structure. Furthermore, we observed that the sheet resistance decreases with the decreases of the sample thickness.
Keywords: SrCuO₂, monolayer copper oxide plane, SrCuO₂ structural transformation, superconductivity